Ferritic chromium steels



11953 w. o. BINDER ET AL 2,624,657l

FERRITIC CHROMIUM STEELS Filed Jan. 19, 1951 3 C -BRITTLE FRACTURE' UDUCTILE FRACTURE llllllllllllllll 2o 25 30 as '40 C HROMIUM INVENTORS 5fTTORNEY Patented Jan. 6, 1 953 FERRITIC CHROMIUM STEELS William O.Binder, Niagara Falls, N. Y., and Bussell Franks, Pittsburgh, Pa.,assignors to Union Carbide and Carbon Corporation, a corporation of NewYork Application January 19, 1951, Serial No. 206,906

6 Claims.

This invention relates to ferritic chromium steels suitable for thefabrication of equipment required to possess good toughness andductility in addition to resistance to corrosion and oxidation.

The so-called "straight ferritic chromium steels have found limitedutility in the past because as chromium is increased to provide necessary resistance to oxidation and other corrosion, the toughness of thesteels at ordinary room temperatures decreases sharply. Thus, althoughsteels containing more than 20% chromium are excellent from thstandpoint of corrosion and oxidation-resistance, they are notchsensitive, and for applications where toughness is required, of theferritic chromium steels only those containing not more than 20%chromium could be used, and generally the chromium content has notexceeded 16%. The notch sensitivity developed by ferritic steelscontaining more than 20% chromium has engaged the attention ofmetallurgists for many years, but no satisfactory solution to theproblem, whether heat treatment or modification of composition, has beenachieved.

It is the principal object of this invention to provide ferriticchromium steels containing 20% to 40% chromium which steels are toughand ductile at ordinary room temperatures and lower temperatures. Morespecifically, it is an object of the invention to provide ferriticsteels containing 20% to 40% chromium which have an Izod impact strengthof at least 18 foot pounds at room temperature. Another object is theprovision of articles fabricated from such steels,

which articles in normal use are required to possess good resistance tocorrosion and oxidation and to retain toughness and ductility.

The invention by means of which these objects are attained is based uponthe discovery that there is a critical relationship in'ferritic chromiumsteels containing 20 to 40% chromiumbetween the toughness of such steelsand the sum of their carbon and nitrogen contents. Further, it has beenfound that as chromium is increased from 20% to 40% the tolerance forcarbon and nitrogen to retain toughness decreases and that therelationship between chrominum and the sum of carbon and nitrogen may berepresented by a curve which separates steels which retain' toughness atroom temperature and below from steels which are brittle at roomtemperature.

The single figure of the drawing is a pair of curves, AB and CD,representing the critical relationship between chromium and the sum ofcar- Mn and nitrogen in ferritic chromium steels containing 20% to 40%chromium.

The invention comprises ferritic chromium steels containing 20% to 40%chromium, carbon and nitrogen in such proportions that their sum for anyselected chromium content when plotted against the chromium content asin the drawing does not fall above the curve, AB, and is preferablybelow the curve CD, the remainder essentially iron and the customaryadditions and impurities ordinarily present in steels of this type. Asis illustrated by the drawing, a steel containing 20% chromium maycontain carbon and nitrogen in a sum not above about 0.06% andbeductile, but a steel containing chromium must contain not more than0.035% carbon and nitrogen to be free of brittleness at roomtemperature, while at chromium the sum of carbon and nitrogen must beleSS than about 0.02%. For applications where it is important to havegreatest impact strength, it is preferred that the composition of asteel to be used fall below the curve, CD. Representative upper limitsfor the sum of carbon and nitrogen under these conditions are 0.04% at20% chromium; 0.025% at 25% chromium and 0.015% at 40% chromium.

The explanation for the rapid decrease in toughness of commercialferritic chromium steels is found in these data which show that thecritical sum of carbon and nitrogen for optimum toughness decreasesrapidly with increasing chromium content and in carbonand nitrogencontents. In

such tests hot-workedsamples of steels were subjected to Izod impacttests at room temperature and at temperatures above and below roomtemperature. The samples were heated for six hours at 900 C. andquenched in water before testing.

Representative examples'of results of these tests are given inthe'following tables.

In; Table I Izod impact values obtained at room temperature, C. and C.are set forth. In Table II are presented tensile test data taken at roomtemperature, tensile strength being measured in pounds per square inch,elongation (per cent E1.) in percentage of a two-inch gage length, and

percent R... A. is percentage reduction of area at fracture;

Table I Composition-Remainder Fe Izod impactft.-lb.

Pep Percent Room l00 6 cecnt 3 other temp. C. C.

20. 0. 013 0. 003 Nil 89 98 1 15/47 21. 79 .014 .002 Nil 92 90 100 23.56 .012 .016 Nil 97 102 9 7/33 25. 44 010 001 Nil 96 100 51 25. 13 014003 Nil 100 100 100 28.17 .018 .002 Nil 90 N.T N. '1 31. 24 012 007 Nil91 N. T N. T 34. .010 .002 Nil 00 91 0 37. 37 .016 004 Nil N. T N. T.25.13 0.01 .014 .003 Nil 100 100 100 25. 70 02 018 003 Nil 98 106 39 83l4 014 002 O. 04 n 100 106 116 26. 41 .23 .018 .012 0. 03 Mn 110 114 662G .24 .016 .003 Nil 100 110 25.18 53 .005 .003 Nil 102 N.T. 5 25. 55 78.005 003 Nil 112 116 110 26.05 1.80 .008 .010 Nil 4 N.T N. T. 25 004 0022 M0 102 N. T N. '1. 25 003 004 1 Ni 93 100 108 25 .006 005 2 Ni 87 9795 25 .004 002 3 Ni 97 86 101 i No Mn or Si added, but traces of bothprobably present.

2 Two specimens with diflcrent values.

N. T. means not tested.

Table IL Composition-Remainder Fe 1 Tensile Percent Percent PercentPercent Percent Percent Strength Cr C N other 25. 44 0. 010 0. 001 Nil01, 600' 43 74. 25. 13 014 003 Nil. 53, 600 37 71. 31. 39 016 002 Nil65, 000 29 31 34175 .010 .002 Nil 40, 000 20 44 37. 77 .016 .004 Nil71,000 33 57 25 003 004 1 Ni 64, 300 27 43 25 006 .005 2 Ni 66, 100 3160 25 004 002 3 Ni 70, 500 I 24 41 1 N0 Mn 01' Si added, but traces ofeach probably present.

their carbon and nitrogen contents is kept below the curves shown in thedrawing. The data show further that by lowering the sum of carbon andnitrogen in such steels below the curve in the drawing the transitionfrom toughness to brittleness is retarded and that in general,

the lower the sum of carbon and nitrogen, the lower the temperature oftransition from toughness to brittleness.

Reduction and control of carbon and nitrogen can be carried out byutilizing materials of very low carbon and nitrogen contents and bymelting and casting the steel at a reduced pressure in contact with anoxygen-bearing material. Under the latter conditions, carbon can beoxidized in preference to chromium and the carbon monoxidev gas whichforms can be readily removed from the steel as. it is not very soluble.The oxidation of carbon, however, only proceeds. if the partial pressureof the. carbon monoxide in the atmosphere over the steel is less thanthe equilibrium value at a given temperature. This means, that to removecarbon from the steel the carbon monoxide gas produced in thecarbonoxygen reaction must be continuously pumped from the system. Asthe process of oxidizing carbon proceeds relatively slowly, suflicienttime must be allowed after the charge is melted for the reaction toapproach equilibrium.

The mechanism of nitrogen removal is best explained by Henrys law whichstates that the.

partial vapor pressure of the solute is proportional to the moi fractionof the solute, which means that the solubility of a gas in a liquiddecreases as the pressure is reduced. The reduction in nitrogen contentof the steel is due to the lowering of the partial pressure of nitrogenover the bath.

To obtain the desired carbon and nitrogen levels it has been foundnecessary to keep the pressure less than about 1000 microns of mercuryif excessive bath temperatures are to be avoided. The use of lowpressures, therefore, has the added advantage of prolonging furnacerefractory life. Excessive sublimation of chromium occurs with longholding at pressures of less than about 50 microns of mercury, andpressures in the range of 200 to 500 microns of mercury give bestoperating results.

Instead of melting the steel at a reduced pressure, the desired controlor reduction of carbon and nitrogen may be attained by maintaining aninert atmosphere free from traces of nitrogen over the steel whilemelting. Argon or helium are ideal for this purpose as they are inertgases, but in using a gas atmosphere it is necessary to circulatepurified gas continuously over the surface of the metal in order toprevent the partial pressure of nitrogen and carbon monoxide frombuilding up in the inert atmosphere.

Small amounts of nickel, copper and cobalt, say up to 3% of nickel orcobalt and 2% copper, may be present in the 20% to 40% ferritic chromiumsteels of the invention without seriously altering their characteristicsprovided that the total carbon and nitrogen content is such that itfalls below th curvesv in the drawing. Likewise, small amounts of othercarbide-forming elements than chromium may be resent, for instance up to3% of molybdenum or tungsten, for improved corrosion resistance orhigh-temperature strength. The steels should be fully deoxidized,manganese and silicon being suitable deoxidizing agents, but for optimumtoughness at low temperatures silicon should fall well below 1%, and thecarbon and nitrogen contents as far below the critical level aspossible. The amount of residual silicon should not greatly exceed thatneeded for deoxidation purposes. Silicon apparently tends to shift thetemperature at which embrittlement occurs upwards, particularly ifcarbon and nitrogen are near the critical total and chromium is inexcess of 25%. Manganese is difficult to retain if the steel is meltedat pressures below about 4 mm. of mercury. However, if the steel is madeunder conditions favorable to the retention of manganese, it issuggested that manganese-not exceed 1 maximum. Sulfur and phosphorus,usual impurities in steel, may be present, but phosphorus tends to shiftthe embrittlement temperature upward, and hence should be kept as low aspossible. Other impurities in the raw materials may have a similarinfluence, and for optimum toughness at low temperatures, the purity ofthe. steel should be kept high.

Thev very low carbon and nitrogen steels covered by'thls invention arecapable of being welded. The steels should be welded under conditionssuch that nitrogen and carbon pick-up is avoided, or with austeniticelectrodes such as 18% chromium- 8% nickel steels or 25% chromium-20%nickel steels as the austenitic steels are not seriously susceptible toloss of toughness due to carbon and nitrogen pick-up during welding, andthey team will produce welds matching the good ductility and toughnessof the base metal.

This application is in part a continuation of our application Serial No.36,558, filed July 2, 1948, now abandoned.

What is claimed is:

l. A ferritic chromium steel containing 20% to 40% chromium, up to 3%nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3%tungsten, the remainder, except for incidental impurities, being iron,carbon and nitrogen, the sum of the carbon and nitrogen contents for aselected chromium content being no greater than the value determined bythe intersection of the plot of the selected chromium content with thecurve AB of the drawing, said steel having in the annealed condition anIzod impact strength of at least 18 foot pounds at room temperature.

2. A ferritic chromium steel containing 20% to 40% chromium, up to 3%nickel, up to 3% cobalt, up to 2% copper, up to 3% molybdenum, up to 3%tungsten, the remainder, except for incidental impurities, being iron,carbon and nitrogen, the sum of the carbon and nitrogen contents for aselected chromium content being no greater than the value determined bythe intersection of the plot of the selected chromium content with thecurve CD of the drawing, said steel having in the annealed condition anIzod impact strength of at least 18 foot pounds at room temperature.

3. An article which in its normal use is required to withstand corrosionand to have high toughness at ordinary room temperatures, said articlebeing composed of a ferritic chromium steel containing 20% to 40%chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3%molybdenum, up to 3% tungsten, the remainder, except for incidentalimpurities, being iron, carbon and nitrogen, the sum of the carbon andnitrogen contents for a selected chromium content being no greater thanthe value determined by the intersection of the plot of the selectedchromium content with the curve AB of the drawing, said steel having inthe annealed condition an Izod impact strength of at least 18 footpounds at room temperature.

4. An article which in its normal use is required to withstand corrosionand to have high toughness at ordinary room temperatures, said articlebeing composed of a ferritic chromium steel containing 20% to 40%chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to 3% 3molybdenum, up to 3% tungsten, the remainder, except for incidentalimpurities, being iron, carbon and nitrogen, the sum of the carbon andnitrogen contents for a selected chromium content being no greater thanthe value determined by the intersection of the plot of the selectedchromium content with the curve CD of the drawing, said steel having inthe annealed condition an Izod impact strength or at least 13 footpounds at room temperature.

5. A welded article which in its normal use i required to withstandcorrosion and to have high toughness at ordinary room temperatures, saidarticle being composed of a ferritic chromium steel containing 20% to40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to3% molybdenum, up to 3% tungsten, the remainder, except for incidentalimpurities being iron, carbon and nitrogen, the sum of the carbon andnitrogen content for a selected chromium content being less than thevalue determined by the intersection of the plot of the selectedchromium content with the curve AB of the drawing, said steel having inthe annealed condition an Izod impact strength of at least 18 footpounds at room temperature.

6. A welded article which in its normal use is required to withstandcorrosion and to have high toughness at ordinary room temperatures, saidarticle being composed of a ferritic chromium steel containing 20% to40% chromium, up to 3% nickel, up to 3% cobalt, up to 2% copper, up to3% molybdenum, up to 3% tungsten, the remainder, except for incidentalimpurities, being iron, carbon and nitrogen, the sum of the carbon andnitrogen contents for a selected chromium content being less than thevalue determined by the intersection of the plot of the selectedchromium content with th curve CD of the drawing, said steel having inthe annealed condition an Izod impact strength of at least 18 footpounds at room temperature.

WILLIAM O BINDER. RUSSELL FRANKS.

REFERENCES CITED The following references are of record in the file ofthis patent:

Transactions, American Society for Metals, vol. 23, page 33, publishedby the American Society for Metals, Cleveland, Ohio.

1. A FERRITIC CHROMIUM STEEL CONTAINING 20% TO 40% CHROMIUM, UP TO 3%NICKEL UP TO 3% COBALT UP TO 2% COPPER, UP TO 3% MOLYBDENUM, UP TO 3%TUNGSTEN, THE REMAINDER, EXCEPT FOR INCIDENTAL IMPURITIES, BEING IRON,CARBON AND NITROGEN, THE SUM OF THE CARBON AND NITROGEN CONTENTS FOR ASELECTED CHROMIUM CONTENT BEING NO GREATER THAN THE VALUE DETERMINED BYTHE INTERSECTION OF THE PLOT OF THE SELECTED CHROMIUM CONTENT WITH THECURVE AB OF THE DRAWING, SAID STEEL HAVING IN THE ANNEALED CONDITION ANIZOD IMPACT STRENGTH OF AT LEAST 18 FOOT POUNDS AT ROOM TEMPERATURE.